1. Field of the Invention
The present invention relates to a fluid dynamic bearing device and a motor equipped with the same.
2. Description of the Related Art
A fluid dynamic bearing device rotatably supports a shaft member by means of a fluid film formed in a bearing gap. Such fluid dynamic bearing devices are roughly classified into ones equipped with a dynamic pressure generating portion for generating dynamic pressure in a lubricating fluid in the bearing gap (so-called dynamic pressure bearings), and ones equipped with no dynamic pressure generating portion. Both types exhibit superior characteristics in terms of high speed rotation, high rotational precision, low noise, etc. In view of those characteristics, a fluid dynamic bearing device is widely suitable for use in, apart from a fan motor mounted in a personal computer (PC), etc., small motors for information apparatuses, for example, a spindle motor mounted in a magnetic disk apparatus such as an HDD and an FDD, an optical disk apparatus such as a CD-ROM, CD-R/RW, and DVD-ROM/RAM, and a magneto-optical disk apparatus such as an MD and an MO, and a polygon scanner motor mounted in a laser beam printer (LBP) or the like.
For example, in a fluid dynamic bearing device for a fan motor, which is among the various motors mentioned above, a rotor with vanes is supported radially by a radial bearing portion so as to be rotatable. Further, a reaction force (thrust force) of the blowing action generated by the vanes is supported by an axial component of a magnetic force generated between a stator coil and a rotor magnet, and a thrust load due to a difference between the magnetic force and the thrust force is supported by a thrust bearing portion. In many cases, in a bearing device for a fan motor, a dynamic pressure bearing is adopted as the radial bearing portion, and a so-called pivot bearing in which the axial end of a rotation shaft is held in contact with a receiving member, is adopted as the thrust bearing portion (see, for example, JP 2000-46057 A).
As an example of a fluid dynamic bearing device for a spindle motor, a structure as shown in FIG. 25 is known. In this fluid dynamic bearing device, there is provided, between the outer peripheral surface of a shaft member 100 and the inner peripheral surface of a bearing member 200 opposed thereto through the intermediation of a radial bearing gap, a radial bearing portion 400 supporting the shaft member 100 radially in a non-contact fashion. Further, there are provided, between the end surfaces of a flange portion 110 provided on the shaft member 100 and members opposed thereto through the intermediation of thrust bearing gaps (bearing member 200 and cover member 300), thrust bearing portions 500 supporting the shaft member in the thrust direction in a non-contact fashion.
In recent years, in particular, in information apparatuses with disk apparatuses incorporated therein, with the rapid progress in performance, a reduction in size and thickness is also being aimed at, and there is an increasingly strict demand for a reduction in the size of a fluid dynamic bearing device. However, in the fluid dynamic bearing device construction as shown in FIG. 25, in which the radial bearing portion and the two thrust bearing portions are stacked together in the axial direction, the axial dimension of the bearing device is generally large, and there are limitations in a reduction in size.
In view of this, there has been disclosed, for example, a construction in which the shaft member is formed in a truncated-cone-like configuration, with a bearing member of a sintered metal being arranged in the outer periphery thereof; there is formed between the shaft member and the bearing member a bearing gap (inclined bearing gap) whose diameter is large on one axial side and small on the other axial side, and a thrust bearing gap is formed between an end surface of the shaft member and a closing member opposed thereto. With this construction, there is no need to provide the flange portion 110 of the shaft member 100 shown in FIG. 25, so it is possible to make the axial dimension of the bearing device so much the smaller (see, for example, JP 2002-276649 A).
In the fluid dynamic bearing device disclosed in JP 2000-46057 A, grooves (dynamic pressure generating portions) for generating fluid dynamic pressure in the radial bearing gap is formed simultaneously with the formation of the bearing member. In this system, however, it is rather difficult to secure a sufficient degree of precision for the dynamic pressure generating portions. Further, since the thrust bearing portions are formed by pivot bearings, wear due to long-term use of the bearing device is inevitable, and there is a fear of this wear adversely affecting the rotational accuracy. Further, the pivot bearing is disadvantageous also from the viewpoint of securing the requisite load capacity (moment rigidity) with respect to moment load.
In the fluid dynamic bearing device disclosed in JP 2002-276649 A, in order to form the inclined bearing gap, it is necessary to form the outer peripheral surface of the shaft member and the inner peripheral surface of the bearing member as conical surfaces. However, it is by no means easy to form the conical surfaces by machining accurately and efficiently. In particular, as compared with the outer peripheral surface, the inner peripheral surface is more difficult to form, so, with the current state of the art, it is rather difficult to finish the conical inner peripheral surface of the bearing member with high accuracy and at low cost; thus, it goes without saying that it is very difficult to secure the requisite precision for the dynamic pressure generating portions when providing the dynamic pressure generating portions such as dynamic pressure grooves, in the conical inner peripheral surface of the bearing member. The bearing performance of a fluid dynamic bearing device, including rotational accuracy, greatly depends on the precision of the bearing gap. Thus, it is impossible to obtain in a stable manner a high precision bearing gap, so a satisfactory bearing performance may not be attained depending upon the design condition, etc.